1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser, and more particularly to a surface emitting semiconductor laser with which oscillation of high-output, fundamental lateral mode light is possible.
The present invention further relates to a surface emitting semiconductor laser. More particularly, the present invention relates to a surface emitting semiconductor laser which can oscillate high-output fundamental lateral mode light, and a structure of an element in which polarization of light is controlled and which has high utility value as a light source employed for optical information processing or optical communication or a light source for a xerographic image forming apparatus.
2. Description of Related Art
In comparison with an edge emitting laser, a vertical cavity surface emitting laser (hereinafter referred to as xe2x80x9cVCSELxe2x80x9d) has numerous merits. For example, manufacturing costs of the VCSEL are low, productivity is high, and achieving a two-dimensional array is easy, coupling efficiency with an optical fiber is high, and electric power consumption is low. For these reasons, the use of the VCSEL for a number of purposes has been investigated in recent years. For example, VCSEL structures, laser characteristics, applications and such VCSEL are described by Kenichi Iga. Fumio Koyama, and Susumu Kinoshita in xe2x80x9cSurface emitting Semiconductor Lasersxe2x80x9d, IEEE Journal of Quantum Electronics, 1988, 24, pp. 1845-1855.
However, fundamental lateral made optical output power of conventional VCSELs remains small, or roughly 1 mW at most. Therefore, the application range of conventional VCSELs has been limited to narrow fields, such as a light pick-up used in a CD-ROM drive. With conventional VCSELs, because laser oscillation by the fundamental lateral mode has been obtained by narrowing the diameter of an optical emission region to roughly several xcexcm, the volume of an active region has been resultantly small, and fundamental lateral mode optical output power has been low.
On the other hand, when the fundamental lateral mode optical output power of the VCSEL increases to 5 mW or more, for example, it becomes possible to use the VCSEL for an image writing apparatus like a laser printer, an optical magnetic disk apparatus and the like.
In Japanese Patent Application Laid-Open (JP-A) No. 10-56233, a VCSEL that has a high-intensity fundamental lateral mode optical output power has been proposed. In this proposal, raising an output of the fundamental lateral mode optical output power is realized by selectively suppressing a laser oscillation condition in a higher-order lateral mode that is secondarily generated in addition to the lateral mode. Namely, fundamental lateral mode oscillation in the VCSEL is generated at the center of an optical waveguide (close to an optical axis), and a higher-order lateral mode oscillation is generated at a remote position separate from the optical axis. Consequently, optical loss of a cavity gradually increases as the distance of separation from the optical axis increases, whereby it becomes possible to suppress a shift to a multi-mode oscillation while increasing an injection current value, and the fundamental lateral mode optical output power can be increased.
As shown in FIG. 12, the VCSEL is structured by a conductive semiconductor substrate 171, a lower DBR (Distributed Bragg Refrector) layer 172, an upper DBR layer 174 having a conductive type opposite to that of the lower DBR layer 172, an active layer region 173 interposed between the lower DBR layer 172 and the upper DBR layer 174, a low reflectance zone 175 formed by ion implantation or the like, a loss determination element 176, and electrodes 177 and 178. A laser beam is emitted along an optical axis 179.
The loss determination element 176 is formed in a concave shape, in order to gradually increase the optical loss of the cavity in accordance with an increase in the distance from the optical axis 179 in a direction orthogonal to the optical axis 179. The concave loss determination element 176 has an operation to refract the cavity laser beam and an operation to either disperse the cavity laser beam sideways or deviate the focus. Consequently, together with increasing the distance from the optical axis 179 in a direction orthogonal to the optical axis 179, the loss determination element 176 can increase the refractive loss and enlarge the optical loss of the cavity. Moreover, the fundamental lateral mode oscillation in the VCSEL is generated close to the optical axis 179, and the higher-order lateral mode oscillation is generated at a remote position separate from the optical axis 179.
As a result, with respect to a higher-order lateral mode, optical loss in a cavity (optical cavity) increases, and a threshold current required for laser oscillation in the higher-order lateral mode increases. On the other hand, with respect to a fundamental lateral mode, variation in the optical loss of the cavity is small, and the threshold current does not vary, thus resulting in an increase of maximum optical output power in the fundamental lateral mode.
Japanese National Publication No. 7-507183 (WO 93/22813) discloses a gain-guiding type surface emitting semiconductor laser that, as shown in FIG. 14, forms a metal contact layer 260 having an optical aperture 265 with a diameter smaller than the diameter of an optical gain region 235, and that suppresses higher-order lateral mode oscillation. With this structure, the optical aperture 265 shields a primary-order lateral mode having a high optical intensity nearer the periphery of the optical aperture than of at the center, or a further higher-order lateral mode having an optical intensity peak at the periphery in addition to at the center, from a fundamental lateral mode having a high optical intensity near the center of the optical aperture 265 within a horizontal surface on a substrate 200. Thus, only the fundamental lateral mode optical output power is selectively taken out, whereby the fundamental lateral mode optical output power is increased.
U.S. Pat. No. 5,753,941 discloses a gain-guiding type surface emitting semiconductor laser. As shown in FIG. 15, an auxiliary layer 38 for lowering cavity optical reflectance is formed beneath an electrode layer 40 used for current injection, whereby higher-order mode generated near the electrode layer of an emission aperture 46 is suppressed. In this structure also, the basic principle is selective suppression of oscillation in a primary-order lateral mode or a further higher-order lateral mode. The method by which suppression is carried out is as follows. Depending on the presence or absence of the auxiliary layer 38, a distribution of optical reflectance is formed within a horizontal surface on a substrate 30. A high reflectance is maintained near the center of the optical aperture 46 within the horizontal plane on the substrate, and a reflectance is effectively lowered near the periphery of the optical aperture 46 by the existence of the auxiliary layer 38. Thus, it becomes easier to oscillate in the fundamental lateral mode by providing a difference in the ease of both oscillations.
As described above, according to the technology disclosed in JP-A No. 10-56233, it becomes possible in principle to raise fundamental lateral mode output. However, at the same time, there are also problems in that a negative influence is exerted on fundamental lateral mode characteristics, and it is considerably difficult to stably form the loss determination element 176 of a predetermined configuration.
As Kenichi Iga and Fumio Koyama describe in xe2x80x9cSurface emitting Laserxe2x80x9d (Ohm, 1990), because it is difficult for the surface emitting semiconductor laser to earn gains necessary for the laser oscillation due to the active region being small, generally a high reflectance is necessary for the cavity. In actuality, VCSEL cavities currently being researched have a reflectance of 99% or higher, when the reflectance of the cavity is low, threshold value current density increases and it becomes difficult for laser oscillation to occur.
In the technology disclosed in JP-A No. 10-56233, the reflectance of the cavity is lowered at a position slightly removed from the optical axis 179. Thus, not only is higher-order lateral mode laser oscillation suppressed, but also laser oscillation of the fundamental lateral mode is suppressed at the same time. As a result, it is surmised that a sufficient high-intensive fundamental lateral mode optical output power will become unobtainable.
Further, the loss determination element 176 is characterized in that it has a curved surface, such as the concave shape shown in FIG. 12, or a convex shape or the like. Therefore, the method by which the configuration of the loss determination element 176 is produced is important. This method is described in detail in JP-A No. 10-56233.
One example will be briefly explained below. As shown in FIG. 13A, a photoresist 182 is coated on a surface of a layer 181 on which a curved surface is to be formed. Next, as shown in FIG. 13B, a cylindrical photoresist pillar 183 is formed using normal exposure, development and bake processings. The photoresist pillar 183 is heated at a temperature of roughly 250xc2x0 C. to 300xc2x0 C. for about five to twenty minutes. Then, as shown in FIG. 13C, the photoresist pillar 183 changes into a layer 184 having convex surface. The temperature of the layer 184 returns to room temperature, and the convex shape of the curved surface is stably maintained thereafter.
Next, a dry etching is administered to this surface using a reactive ion etching (RIE) from the top. The layer 184 works as an etching mask and reflects this shape. As a result, a structure 185 having a curved surface with a convex configuration is formed, as shown in FIG. 13D.
A method of forming a structure having a curved surface with a convex configuration has been described above. However, when the photoresist pillar 183 is provided at the periphery of the layer 181 instead of at the center of the layer 181, it is possible to form a structure having a concave surface with a concave configuration at the center of the layer 181.
The shape of the layer 184 that operates as an etching mask is required to have a predetermined curved surface at a predetermined position. However, even based on present etching technologies, it is substantially difficult to form with good reproducibility and without positional dependency a curved surface at always the same position. This problem becomes particularly pronounced when a large number of VCSEL elements is provided to form a two-dimensional array.
Moreover, it is extremely difficult to finish the etching at a proper position either at the point the layer 184 working as the etching mask in the RIE process disappears or thereafter, in order to form the loss determination element 176 having a predetermined, curved-surface configuration and a predetermined film thickness.
In addition, when a large number of VCSEL elements is provided to form a two-dimensional array, it is extremely difficult to control with high precision an etching selection ratio between materials that structure the photoresist pillar 183 and the loss determination element 176 on the same substrate or on different substrates. Therefore, it is extremely difficult to match reflectance characteristics of the loss determination element 176 between each of the VCSEL elements.
As described above, it is extremely difficult to eliminate or minimize variations in the shape and film thickness of the loss determination elements 176 between VCSEL elements on the same substrate, between VCSEL elements on different substrates, or between VCSEL elements in which lots in processes are different.
On the other band, the optical loss of the cavity is gradually increased as the cavity is positioned farther from the optical axis 179 by utilizing the concave shape of the curved surface of the loss determination element 176. Accordingly, the injection current value is increased, the higher-order lateral mode shift to laser oscillation is suppressed, and fundamental lateral mode laser oscillation is made possible. Therefore, when the shape of the concave surface of the loss determination element 176 is different, the higher-order lateral mode optical output power value of the VCSEL that shifts to the laser oscillation is different, i.e., a fundamental lateral mode maximum optical output power value, is different. As a result, fundamental lateral mode maximum optical output power values of the VCSEL elements are different between VCSEL elements on the same substrate, between VCSEL elements on different substrates, or between VCSEL elements in which lots in processes are different. Therefore, it is difficult to industrially utilize for purposes in which a high-intensity fundamental lateral mode optical output power is required the technology disclosed in JP-A No. 10-56233.
With regard to the characteristics of a structure of a surface emitting semiconductor laser, Japanese National Publication No. 7-507183 (WO 93/22813) discloses xe2x80x9cthe metal layer has an optical aperture aligned in a direction vertical to a gain region, and this optical aperture has a diameter equal to or smaller than the diameter of the gain areaxe2x80x9d.
Here, the reason that the optical aperture has a diameter equal to or smaller than the diameter of the gain area in order to sufficiently suppress higher-order lateral mode oscillation is that the diameter of the optical aperture is, is not irrelevant to the fact that the structure of the elements indicated in the embodiments uses a technology to raise the resistance of a semiconductor layer by a proton injection.
In a surface emitting semiconductor laser of a proton injection system, the conductivity of a region subjected to proton injection is lowered in comparison with the conductivity of a region that has not been subjected to proton injection. Accordingly, a current contracted structure is formed, and a carrier injected from upper and lower electrodes intensively passes through a specific portion (the region that has not been subjected to proton injection) within the planar surface of an active region. Consequently. recombination of electrons with positive holes occurs at this region, photons are generated, and the photons propagate within the cavity to result in laser oscillation. A laser having such a structure is generally called a gain-guiding laser. That is, a gain-guiding laser denotes a structure in which a region (an optical gain region) where the recombination of electrons with positive holes occurs actively is limited by proton injection to result in laser oscillation.
The proton injection technique has conventionally been utilized in semiconductor processes, and may be regarded as an established process. The proton injection technique has also been utilized in the manufacturing process of surface emitting semiconductor lasers since their initial period. However, because of the nature of the technique, in that a large amount of impure ions that become foreign materials to the semiconductor material to be processed is implanted, it is difficult to accurately demarcate the interface between the implanted region and other regions, and removing indistinct regions is unavoidable. Therefore, often the diameter of the non-injected region is, albeit narrow, 10 xcexcm and typically around 20 xcexcm.
At the time the diameter of the optical gain region that has been defined by the current confining structure is 10 to 20 xcexcm in the gain-guiding type surface emitting semiconductor laser, when the current density is raised by increasing the amount of carriers injected to obtain a large optical output power, ununiformity in the distribution of the carrier generally occurs so that the lateral mode easily becomes unstable. Further, near the center of the optical gain region in which the recombination of carriers occurs most actively, carrier consumption amount becomes larger than that in surrounding regions, whereby a so-called hole-burning phenomenon, in which there is a shortage of positive holes, occurs. Accordingly, a state in which the lateral mode breaks up and oscillation in the lateral mode becomes difficult is triggered.
In Japanese National Publication No. 7-507183, which was devised to avoid this problem, it is essential that the diameter of the optical aperture that meets the object of selectively taking out only the fundamental lateral mode optical output power by shielding the higher-order lateral mode with the optical aperture is xe2x80x9ca diameter equal to or smaller than the diameter of the gain areaxe2x80x9d. The embodiments disclose xe2x80x9ctypically, the diameter of the optical aperture 265 is from 2 xcexcm to 7 xcexcm, and the diameter of the optical gain region 235 is from 10 xcexcm to 30 xcexcmxe2x80x9d. The range of these numerical values is consistent with the above description.
The above method has achieved certain results regarding an increase in the fundamental lateral mode optical output power. However, speaking in relation to the gain-guiding type surface emitting semiconductor laser, there have been the following substantial problems. Electrical power consumption of the gain-guiding surface emitting semiconductor laser is about the same as or slightly lower than that of the edge emitting laser. Emission efficiency is about 20%, which is not too high. Further, though detailed explanation cannot be given here, for reasons of principle, the optical response property is extremely slow (msec order) unless a certain bias voltage is applied. Therefore, recently, the gain-guiding surface emitting semiconductor laser is being superseded by a surface emitting semiconductor laser of a selective oxidization system to be described later.
On the other hand, U.S. Pat. No. 5,753,941 discloses a surface emitting semiconductor laser provided with xe2x80x9can electrode layer 40 formed on the second reflector layer 36 and constituting a cavity for emitting light transmitted through the second reflector layer 36, wherein the electrode layer 40 comprises a metal layer 44 having a high electrical conductivity and connected to an external power source, and a conductive auxiliary reflector layer 42 formed under said metal layer, and having a reflectivity which is lower than those of the first reflector layer 32 and the second reflector layer 36xe2x80x9d.
Here, reasons why the electrode layer 40 has a two layer structure comprising the metal layer 44 and the auxiliary reflection layer 42, and has a reflectivity which is lower than those of the first reflector layer 32 and the second reflector layer 36 are as follows. When the electrode layer 40 has a structure comprising only the metal layer 44 without the auxiliary reflection layer 42, a reflection beam from the electrode layer 40 returns in the direction of the second reflector layer 36, thereby impacting conditions for fundamental lateral mode oscillation and emission intensity. Further, when the electrode layer 40 has a structure in which the auxiliary reflection layer 42 has been introduced, the auxiliary reflection layer 42, having a low reflectivity and present at a position nearer the higher-order mode having a higher optical intensity at the periphery rather than at the center of the emission aperture 46, works in a direction to suppress higher-order mode oscillation.
The principle of causing a distribution of reflectivity near the aperture 46, from which a laser beam is emitted, and influencing lateral mode properties by providing the auxiliary reflection layer 42 having a relatively low reflectance can be understood intuitively. However, in considering that the order of a lateral mode whose existence inside the aperture is permitted by the diameter of the optical gain region or by the distribution of the injected carrier changes, it becomes necessary to specify the portion whose reflectivity is to be lowered, in order to suppress higher-order mode oscillation by influencing the change. In other words, it is insufficient to provide no numerical disclosures regarding what settings are effective for the length of the protruding portion 43 or the distance thereof from the aperture center. Without an indication of preferable ranges of condition, questions linger as to whether the fundamental lateral mode can actually be obtained.
Further, the VCSELs described in Japanese National Publication No.7-507183 and U.S. Pat. No. 5,753,941 are both gain-guiding type VCSELs. Even in the embodiments therein, concrete descriptions are provided that mainly use proton injection type surface emitting semiconductor lasers as examples. Therefore, higher-order mode suppression assuming a refractive index guiding type surface emitting semiconductor laser that uses the selective oxidization technique and that is now emerging as the mainstream of surface emitting semiconductor lasers has not been taken into consideration. Thus, it can be predicted that, even adapting these means as they are to the surface emitting semiconductor laser of the selective oxidization technique, effects cannot be adequately demonstrated.
In the application of laser, use of circular light-emitting spots is often desired. In this respect, the surface emitting semiconductor laser can form a circular spot since it does not require any extreme aspect ratio in a light-emitting region. Further, since two-dimensional integration in the same wafer and inspection of elements at a wafer level are enabled, the surface emitting semiconductor laser can provide a laser element which has extremely high density and which can be manufactured inexpensively as compared to known semiconductor lasers. As a result, demand for surface emitting semiconductor lasers in the field of communication is increasing.
Ordinarily surface emitting semiconductor lasers except surface emitting semiconductor lasers with a strained quantum well active layer do not have anisotropy of gain in planes perpendicular to the light-emitting directions, and have a probability of homogeneously polarizing in all directions.
When polarization characteristics (directions of polarized wave planes) are different for each element, if the surface emitting semiconductor lasers are used together with optical elements having polarized waves-dependency, such as mirrors or polarized light beam splitters to which no specified coatings have been applied, optical characteristics (here, optical output powers) of light beams which have passed through these optical elements vary. For this reason, in engineering developments of the surface emitting semiconductor lasers, various improvements for stabilizing polarization of laser elements in one fixed direction have conventionally been attempted.
Japanese Patent Application Laid-Open (JP-A) No. 6-302911 discloses a surface emitting semiconductor laser in which a laser gain region is appropriately formed, and more specifically, a cross section of the gain region is configured so as to have an aspect ratio of more than 1.2, whereby devices are polarized in a longer dimension in the fundamental mode. The active region of this surface emitting semiconductor laser is formed in a specific shape (e.g. an irregular rhomboid, dumbbell, or cross shape) in which a primary dimension of two cross sections orthogonal to each other is larger than a sub dimension by a coefficient of 1.2 or more. Light emitted from the active region formed in such a shape is preferably provided with a low order mode, and is polarized (instead of xe2x80x9cpolarizationxe2x80x9d, a term xe2x80x9cdeflectionxe2x80x9d is used in this specification by mistake) in a direction of a primary dimension AC.
This disclosure might be a simple polarization controlling method because this cross-sectional configuration can be formed by forming a cavity into a specified configuration by dry-etching in a refractive index guide laser or by forming a shaped-ion implantation region around a gain-guiding laser.
However, no description of basic principle has been made in the specification of the JPA-No. 6-302911 as to why polarization directions can be controlled and at the same time, an oscillation in the fundamental mode is selected when xe2x80x9cthe cross-section of the gain region is configured so as to have the aspect ratio of more than 1.2xe2x80x9d. Besides, a method in which this has been verified is not based on direct evaluation of the polarization characteristics, thus making it difficult to objectively judge the effects of this disclosure.
In IEEE Photonics Technology Letters (Volume No. 10, pp. 633 (1998)), a description is made of indium gallium arsenide (InGaAs)-selective oxidation type surface emitting laser using an off-angled substrate in which a gallium arsenide (GaAs) (311) B-surface is used as a normal direction. The diameter of an optical guided wave (active) region that is formed by a selective oxidation is a rectangle of 6 xcexcmxc3x973 xcexcm. A long axial direction of the rectangle corresponds to [xe2x88x92233] direction and a polarization direction thereof also corresponds to [xe2x88x92233] direction. This surface emitting semiconductor laser could obtain polarization characteristics in which orthogonal polarization suppression ratio is equal to or greater than 30 dB by using the off-angled substrate having a specific surface orientation which is referred to as (311) B together with a strained quantum well active layer.
This method seems to be very effective means for controlling polarization. However, since the GaAs off-angled substrate whose (311) B-surface is the normal direction is formed according to special specifications, a manufacturing cost of the GaAs off-angled substrate is higher than that of the GaAs substrate which is most frequently used and whose (100) surface is the normal direction. Further, when a crystal growth is conducted on this off-angled substrate, it is presumed that various conditions such as temperature, gas flow, and the like are different greatly from those of a crystal growth conducted on (100) surface which crystal growth is widely conducted, thus requiring an extraordinary cost and time until crystal growth conditions that can exhibit high reproducibility are found and put into practical use.
An object of the present invention is to provide a refractive index waveguide type surface emitting semiconductor laser that can be manufactured easily and that is capable of oscillating fundamental lateral mode light having a high output.
Another object of the present invention is to provide a surface emitting semiconductor laser which can control polarization of a laser beam in a fixed direction with a relatively simple structure and which has excellent electric and optical characteristics such as low threshold current and high output.
As a result of their extensive investigation, the inventors of the present invention have found that, in accordance with the following means, a high-output surface emitting semiconductor laser in which higher-order lateral mode oscillation is suppressed without exerting a negative effect on fundamental lateral mode oscillation is obtainable.
A first aspect of the present invention is a surface emitting semiconductor laser comprising: a semiconductor substrate having sequentially layered thereon a lower multi-layer mirror, an active layer region, and an upper multi-layer mirror that, together with the lower multi-layer mirror, contributes to the formation of a cavity; an upper electrode disposed on an upper layer of the upper multi-layer mirror and provided with an aperture that forms an emission region of a laser beam generated at the active layer region; and a current confinement portion disposed between the upper electrode and the lower multi-layer mirror and provided with an aperture that forms a current path; wherein, on the basis of a reflectance of a region of the cavity corresponding to the upper electrode, an aperture diameter of the upper electrode and an aperture diameter (an oxide aperture diameter) of the current confinement portion are determined such that a difference between an optical loss of the cavity in a higher-order lateral mode of the laser beam and an optical loss of the cavity in a fundamental lateral mode of the laser beam becomes a value in the vicinity of a maximum value and such that the higher-order lateral mode is suppressed.
Ordinarily, optical loss of a cavity in the higher-order lateral mode of a laser beam becomes larger than optical loss of the cavity in the fundamental lateral mode of a laser beam. Difference in optical loss of the cavity here means the difference when the optical loss of the cavity in the fundamental lateral mode of a laser beam is subtracted from the optical loss of the cavity in the higher-order lateral mode of a laser beam. The larger the difference in optical loss of the cavity is, the more preferable. Determining the aperture diameter of the upper electrode and the aperture diameter of the current confinement portion so that the difference in optical loss becomes a value in the vicinity of a maximum value is more preferable.
Further, the reflectance of the cavity of the region corresponding to the upper electrode is a reflectance of a cavity structured to include the region in which the upper electrode is provided directly on the upper multi-layer mirror. The reflectance of the cavity of the region corresponding to the emission region is a reflectance of a cavity structured to include the region that becomes the emission region of the upper multi-layer mirror.
In the surface emitting semiconductor laser relating to the first aspect, it is possible to enlarge a rate at which the aperture diameter of the upper electrode is increased as the reflectance of the cavity of the region corresponding to the upper electrode is lowered. Further, it is possible to make the aperture diameter of the upper electrode equal to or larger than the aperture diameter of the current confinement portion. Yet further, it is possible to ensure that the reflectance of the cavity of the region corresponding to the upper electrode becomes lower than the reflectance of the cavity of the region corresponding to the emission region.
A second aspect of the present invention is a surface emitting semiconductor laser comprising: a semiconductor substrate having sequentially layered thereon a lower multi-layer mirror, an active layer region, and an upper multi-layer mirror that, together with the lower multi-layer mirror, contributes to the formation of a cavity; an upper electrode disposed on an upper layer of the upper multi-layer mirror and provided with an aperture that forms an emission region of a laser beam generated at the active layer region; and a current confinement portion disposed between the upper electrode and the lower multi-layer mirror and provided with an aperture that forms a current path; wherein, on the basis of a reflectance of a region of the cavity corresponding to the emission region and a reflectance of the cavity of a region corresponding to the upper electrode, an aperture diameter of the upper electrode and an aperture diameter of the current confinement portion are determined such that a difference between an optical loss of the cavity in a higher-order lateral mode of the laser beam and an optical loss of the cavity in a fundamental lateral mode of the laser beam becomes a value in the vicinity of a maximum value and such that the higher-order lateral mode is suppressed.
In the surface emitting semiconductor laser relating to the second aspect, it is possible to enlarge a rate at which the aperture diameter of the upper electrode it increased as the reflectance of the cavity in the region corresponding to the upper electrode is lowered. Further, it is possible to make the aperture diameter of the upper electrode equal to or larger than the aperture diameter of the current confinement portion. Yet further, it is possible to ensure that the reflectance of the cavity of the region corresponding to the upper electrode becomes lower than the reflectance of the cavity corresponding to the emission region. Still yet further, it is possible to increase the aperture diameter of the upper electrode as the reflectance of the cavity of the region corresponding to the emission region is lowered, and to reduce the aperture diameter of the upper electrode when the reflectance of the cavity of the region corresponding to the emission region is made higher.
A third aspect of the present invention is a surface emitting semiconductor laser comprising: a semiconductor substrate having sequentially layered thereon a lower multi-layer mirror, an active layer region, and an upper multi-layer mirror that, together with the lower multi-layer mirror, contributes to the formation of a cavity; an upper electrode disposed on an upper layer of the upper multi-layer mirror and provided with an aperture that forms an emission region of a laser beam generated at the active layer region; and a current confinement portion disposed between the upper electrode and the lower multi-layer mirror and provided with an aperture that forms a current path; wherein, an aperture diameter of the upper electrode and an aperture diameter of the current confinement portion are determined such that a difference between an optical loss of the cavity in a higher-order lateral mode of the laser beam and an optical loss of the cavity in a fundamental lateral mode of the laser beam becomes a value in the vicinity of a maximum value and such that the higher-order lateral mode is suppressed.
In the surface emitting semiconductor laser relating to the third aspect, it is possible for the aperture diameter of the upper electrode to be determined at a value that suppresses a higher-order lateral mode, and for the aperture diameter of the current confinement portion to be determined at a value that permits a higher-order lateral mode of a third order or lower. Further, when the aperture diameter of the upper electrode is to be made larger than the aperture diameter of the current confinement portion, the aperture diameter of the upper electrode can be made larger within a range of about 2 xcexcm or lower, and when the aperture diameter of the upper electrode is to be made smaller than the aperture diameter of the current confinement portion, the aperture diameter of the upper electrode can be made smaller within a range of about 1 xcexcm or lower, still further, the aperture diameter of the current confinement portion is preferably about 3 xcexcm to about 5 xcexcm.
It is possible to ensure that optical loss of the cavity in the higher-order lateral mode of a laser beam becomes larger than the optical loss of the cavity in a basic lateral mode of a laser beam, and to ensure that the reflectance of the cavity of the region corresponding to the upper electrode becomes lower than a reflectance of the cavity of the region corresponding to the emission region. The reflectance of the cavity of the region corresponding to the upper electrode is preferably 95% or lower, and more preferably 80% or lower.
Methods for lowering the reflectance of the cavity by providing the upper electrode include the methods described below. According to these methods, the reflectance of the cavity of the region corresponding to the upper electrode can be lowered without providing a reflectance lowering structure having a special shape. As a result, manufacturing is easy.
(1) A method in which the upper electrode is formed by layering two or more kinds of metal materials.
(2) A method in which the upper electrode is formed by forming a thin film made of a metal material, heat treating the thin film in a temperature range of 250xc2x0 C. to 400xc2x0 C., and advancing an alloying between the thin film and a layer adjacent to the thin film.
It is preferable to select metal materials for structuring the upper electrode from Au, Pt, Ti, Zn, Ni, In, W, Cu, Al, Auxe2x80x94Sn alloy, Auxe2x80x94Zn alloy, Auxe2x80x94Ge alloy, and indium tin oxide (ITO). The thin film made of a metal material may be formed by metal deposition.
It is preferable that the heat treatment is carried out at a temperature within a temperature range of 300xc2x0 C. to 350xc2x0 C. The heat treatment method is preferable administered in accordance with any one of methods selected from flash lamp annealing by infrared rays, laser annealing, micro-wave heating, electron beam annealing, and lamp heating.
The current confinement portion may be formed by insulating a periphery of a current path through oxidization and air gap by etching.
Further, according to a further aspect of the present invention, there may be provided a surface emitting semiconductor laser comprising: a semiconductor substrate having a lower multi-layer mirror, an active layer region and an upper multi-layer mirror sequentially formed on an upper portion, and having a lower electrode provided on a lower portion; an upper electrode as an upper layer of the upper multi-layer mirror, provided to surround an emission aperture around the periphery of an emission center of a laser beam generated in the active layer region, and forming a pair with the lower electrode and made of a metal material for injecting a current into the active layer region; and a current confinement portion provided between the upper electrode and the lower electrode, and formed to insulate a peripheral edge portion of a current path, wherein the reflectance of a multi-layer mirror at the periphery of the emission center on which the upper electrode has been provided is made lower than the reflectance of a multi-layer mirror at the emission center, and the diameter of the emission aperture is made larger than the diameter of the current confinement portion according to a level of reduction in the reflectance of the multi-layer mirror at the periphery of the emission center.
A surface emitting semiconductor laser according to a fourth aspect of the present invention, comprises: a semiconductor substrate having sequentially layered thereon a lower multi-layer mirror, an active layer region, and an upper multi-layer mirror that, together with the lower multi-layer mirror, contributes to the formation of a cavity; an upper electrode disposed on an upper layer of the upper multi-layer mirror and provided with an aperture that forms an emission region of a laser beam generated at the active layer region; and a current confinement portion disposed between the upper electrode and the lower multi-layer mirror and provided with an aperture that formed a current path, wherein an aperture diameter of the upper electrode and an aperture diameter of the current confinement portion are determined such that a difference between an optical loss of the cavity in a higher-order lateral mode of the laser beam and an optical loss of the cavity in a fundamental lateral mode of the laser beam becomes a value in the vicinity of a maximum value, and the higher-order lateral mode is suppressed, and at least one of the aperture formed in the upper electrode and the aperture of the current confinement portion is formed into a two-fold symmetrical configuration having long and short axes with respect to arbitrary two axial directions orthogonal to each other in a plane.
In the surface emitting semiconductor laser according to the fourth aspect of the present invention, the other of the aperture formed in the upper electrode and the aperture of the current confinement portion is preferably isotropic to arbitrary two axial directions orthogonal to each other in the plane. Further, the isotropic configuration is preferably one of a circle, a square, and a regular polygon.
The surface emitting semiconductor laser according to the fourth aspect of the present invention is characterized by that one configuration of the aperture formed in the upper electrode and the aperture of the current confinement portion is formed into a symmetrical configuration, and the other configuration is a two-fold symmetrical geometric configuration having long and short axes with respect to arbitrary two axial directions which are orthogonal to each other in the plane of the substrate. The surface emitting laser having this structure has characteristics of arranging a plane of polarization of a laser beam in the direction of the long axis thereof, thus making it possible to control the plane of polarization of the laser beam in a fixed direction.
In the surface emitting semiconductor laser according to the fourth aspect of the present invention, if the crystal orientation of the semiconductor substrate is (100) surface, it is preferable that the arbitrary two axial directions is a combination of [01-1] direction and [011] direction or a combination of directions which are crystallographically isotropic to the combination of [01-1] direction and [011] direction.
Further, it is preferable that the two-fold symmetrical configuration having the long and short axes is formed in a flat oval, ellipse, rectangular, or rhomboidal shape.
The principle of the fundamental lateral mode optical output power according to the present invention will be explained below. In a surface emitting semiconductor laser shown in FIG. 1, an aperture diameter of an aperture of a current confinement portion 24 is defined as an aperture diameter (Woxide) of the current confinement portion, and a diameter of an electrode aperture 27 is defined as an metal (electrode) aperture diameter (Wmetal). Then, the aperture diameter of the current confinement portion and the metal aperture diameter are changed variously, and a difference between a round-trip loss of a cavity in a fundamental lateral mode and a normalized round-trip loss is calculated. The surface emitting semiconductor laser shown in FIG. 1 has the same structure as the structure (shown in FIG. 5G) of a surface emitting semiconductor laser in a first embodiment to be described later, except the reflectance of a multi-layer mirror, the aperture diameter of a current confinement portion, and the value of an metal aperture diameter are different respectively. A pair of DBR layers constitute a cavity. In FIG. 1, parts that are identical to those of the surface emitting semiconductor laser shown in FIG. 5G are attached with like reference symbols, and their explanation will be omitted.
In FIG. 2, the round trip loss of a cavity in a fundamental lateral mode (zero-order mode) is expressed as a function of the metal aperture diameter when the aperture diameter of the current confinement portion is constant at 3.5 xcexcm. Based on the assumption that a reflectance (Rcavity) of the cavity of the region corresponding to the emission region is 99.4%, a reflectance (Rmetal) of the cavity of the region corresponding to the upper electrode is changed from 75% to 99%. Further, the value of the metal aperture diameter for each reflectance (Rmetal) is changed from 1.5 to 6.0 xcexcm. The increase in the round trip loss works in a direction to make it difficult to generate an oscillation. As can be understood from the graph, when only the round trip loss of the cavity in the fundamental lateral mode is looked at, the reduction of the reflectance of the cavity of the region corresponding to the upper electrode increases the loss in the fundamental lateral mode. Therefore, in terms of fundamental lateral mode oscillation, it is preferable that the reflectance of the cavity of the region corresponding to the upper electrode is high and that the metal aperture diameter is large.
Here, behavior in the higher-order mode becomes a problem. When the electrode aperture is made larger, this works in a direction to facilitate the oscillation in a higher-order mode where the optical intensity is higher at the periphery of the emission aperture that at the center of the emission aperture. Therefore, it is necessary to investigate which one of the influence to the fundamental lateral mode and the influence to the higher-order mode becomes larger when the metal aperture diameter is made larger.
FIG. 3 shows how a value obtained by dividing a difference between the round trip loss of the cavity in the primary-order lateral mode and the round trip loss of the cavity in the fundamental lateral mode by the round trip loss of the cavity in the fundamental lateral mode (hereinafter to be referred to as a xe2x80x9cnormalized round trip loss differencexe2x80x9d) changes based on the metal aperture diameter, when the reflectance of the cavity of the region corresponding to the upper electrode, with the electrode formed above, is changed from 75% to 99%. This normalized round trip loss difference is obtained under the assumption that the aperture diameter of the current confinement portion is constant at 3.5 xcexcm, and that the reflectance of the cavity of the region corresponding to the emission region is 99.4%. In this case, the increase in the normalized round trip loss difference means that the ratio of the increase in the round trip loss in the primary-order lateral mode is larger than the ratio of the increase in the roundtrip loss in the fundamental lateral mode. In this case, the oscillation in the primary-order lateral mode is relatively hard to occur, and as a result, the oscillation in the fundamental lateral mode becomes more advantageous than in the primary-order lateral mode. It is possible to expand the concept of this primary-order lateral mode to a higher-order lateral mode, and the increase in the normalized round trip loss difference also means that the oscillation in the fundamental lateral mode is more advantageous than in the higher-order lateral mode.
In actuality, the laser oscillation between in the fundamental lateral mode and in the higher-order lateral mode is not selected based on only the relative ratio of the round trip loss. Therefore, both the fundamental lateral mode and the higher-order lateral mode exist together. However, as shown in FIG. 3, the normalized round trip loss difference in each reflectance (Rmetal) has a peak value in a predetermined metal aperture diameter. When an metal aperture diameter near the metal aperture diameter corresponding to this peak value is utilized, the oscillation in the fundamental lateral mode becomes more advantageous than in the higher-order lateral mode. In other words, it can be understood that the higher-order lateral mode is suppressed.
From FIG. 3, it can also be understood that the metal aperture diameter that shows a peak and the steepness of the peak change according to the reflectance of the cavity of the region corresponding to the upper electrode. For example, when the reflectance of the cavity of the region corresponding to the upper electrode is 95%, the normalized round trip loss difference becomes a maximum at the time the metal aperture diameter is 3.3 xcexcm. In other words, 3.3 xcexcm is an optimum metal aperture diameter. When the reflectance of the cavity of the region corresponding to the upper electrode is 90%, the normalized round trip loss difference becomes a maximum when the metal aperture diameter is 4.0 xcexcm. When the reflectance of the cavity of the region corresponding to the upper electrode is 85%, the normalized round trip loss difference becomes a maximum when the metal aperture diameter is 4.2 xcexcm. When the reflectance of the cavity of the region corresponding to the upper electrode is 80%, the normalized round trip loss difference becomes a maximum when the metal aperture diameter is 4.6 xcexcm. When the reflectance of the cavity of the region corresponding to the upper electrode is 75%, the normalized round trip loss difference becomes a maximum when the metal aperture diameter is 4.8 xcexcm. Therefore, it is preferable that the metal aperture diameter is within the range from 3.3 xcexcm to 4.8 xcexcm when the reflectance of the cavity of the region corresponding to the upper electrode is within the range from 75% to 95%.
When the reflectance of the cavity of the region corresponding to the upper electrode is 95%, the steepness of the peak of the normalized round trip loss difference becomes high. As the reflectance of the cavity of the region corresponding to the upper electrode becomes lower toward 90%, 85%, 60% and 75%, the peak becomes steeper, and the normalized round trip lose difference increases. In other words, when the reflectance of the cavity of the region corresponding to the upper electrode is lowered to 90%, 85%, 80% and 75% respectively, the oscillation in the fundamental lateral mode becomes more advantageous than in the higher-order lateral mode. When the reflectance of the cavity of the region corresponding to the upper electrode becomes lower than 75%, both the higher-order lateral mode and the fundamental lateral mode are suppressed, and the optical output power decreases as a result.
Table 1 shows optimum values of Wmetal predicted from the above result when each value of Rcavity, Rmetal and Woxide has been determined. In example 5 to example 7, each value of Rcavity, Rmetal, Woxide and Wmetal is an actually measured value.
FIG. 7 shows preferable ranges of Wmetal from the above result. As shown in FIG. 7, when the reflectance (Rcavity) of the cavity of the region corresponding to the emission region is within the range from 99.0% to 99.7% and also when the reflectance (Rmetal) of the cavity of the region corresponding to the upper electrode is within the range from 75% to 95%, it is preferable that the metal aperture diameter (Wmetal) is within the range from 3.0 xcexcm to 5.0 xcexcm for the aperture diameter (Woxide) of the current confinement portion at 3.0 xcexcm. Further, it is preferable that the metal aperture diameter (Wmetal) is within the range from 3.2 xcexcm to 5.2 xcexcm for the aperture diameter (Woxide) of the current confinement portion at 3.5 xcexcm. It is preferable that the metal aperture diameter (Wmetal) is within the range from 3.5 xcexcm to 5.5 xcexcm for the aperture diameter (Woxide) of the current confinement portion at 4.0 xcexcm. It is preferable that the metal aperture diameter (Wmetal) is within the range from 4.0 xcexcm to 6.0 xcexcm for the aperture diameter (Woxide) of the current confinement portion at 4.5 xcexcm.
Further, in the metal aperture diameter near the optimum metal aperture diameter, the normalized round trip loss difference is not largely different from that when the metal aperture diameter takes the optimum value. Therefore, there is a predetermined permissible range of metal aperture diameter. In the following cases, it is possible to obtain an effect approximately similar to that when the metal aperture diameter is taken. When the metal aperture diameter is made larger than the aperture diameter of the current confinement portion, the metal aperture diameter is increased to a range of the aperture diameter of the current confinement portion plus 0 to 2 xcexcm. When the metal aperture diameter is made smaller than the aperture diameter of the current confinement portion, the metal aperture diameter is decreased to a range of the aperture diameter of the current confinement portion minus 0 to 1 xcexcm.
FIG. 4 shows how a normalized round trip loss difference changes when the reflectance of the cavity of the region corresponding to the mission region is changed to 99.0%, 99.4%, and 99.7%, by assuming that the aperture diameter of the current confinement portion is constant at 3.5 xcexcm, and the reflectance of the cavity of the region corresponding to the upper electrode, having an electrode formed in the above, is 90%. As shown in FIG. 4, it can be understood that when the reflectance of the cavity of the region corresponding to the emission region becomes high, the metal aperture diameter at which the normalized round trip loss difference shows a peak value becomes slightly large. It is preferable that the metal aperture diameter is within a range from 3.8 xcexcm to 4.2 xcexcm when the reflectance of the cavity of the region corresponding to the emission region is within the range from 99.0% to 99.7%. Further, when the reflectance of the cavity of the region corresponding to the emission region becomes higher, the oscillation in the fundamental lateral mode becomes more advantageous than in the higher-order lateral mode. A variation curve is indicated in FIG. 3 by a broken line for a case in which the reflectance of the cavity of the region corresponding to the upper electrode is 90% and the reflectance of the cavity of the region corresponding to the emission region is 99.0%, and a case in which the reflectance of the cavity of the region corresponding to the upper electrode is 90% and the reflectance of the cavity of the region corresponding to the emission region is 99.7%.
FIG. 3 shows by broken lines the curves of changes in a difference between normalized round trip losses in the case in which the reflectance of the cavity in a region corresponding to the upper electrode is 90% and the reflectance of the cavity in a region corresponding to the light-emitting region is 99.9%, and in the case in which the reflectance of the cavity in a region corresponding to the upper electrode is 90% and the reflectance of the cavity in a region corresponding to the light-emitting region is 99.7%.
Therefore, it is possible to efficiently obtain the fundamental lateral mode oscillation by optimizing the metal aperture diameter for the aperture diameter of the current confinement portion according to the reflectance of the cavity of the region corresponding to the upper electrode and the reflectance of the cavity of the region corresponding to the emission region. While it is not possible to extract an optical beam when the reflectance of the cavity of the region corresponding to the emission region is 100%, it is difficult to obtain the laser oscillation when the reflectance of the cavity at the center of the emission aperture is 95% or below. As the reflectance of the cavity at the center of the emission aperture is usually 99% or above, it is substantially possible to efficiently obtain the fundamental lateral mode oscillation by optimizing the metal aperture diameter for the aperture diameter of the current confinement portion according to the reflectance of the cavity of the region corresponding to the emission region.
As explained above, according to the surface emitting semiconductor laser of the present invention, it is possible to increase the fundamental lateral mode output by selectively suppressing a laser oscillation condition in a higher-order lateral mode that is secondarily generated, without losing the characteristics of the fundamental lateral mode oscillation.